1. Field of the Invention
The present invention relates to a storage device and the control method thereof.
2. Description of the Related Art
In recent years, the reduction of the total cost of ownership (TCO) of a storage system becomes increasingly important in information business sites such as a data center. On the other hand, demand for recording data reliably for a long period of time is increasing. As an example of this fact, the document data of financial institutions, medical institutions, etc., are required to be stored without being erased by law. Under these circumstances, there is much demand for highly reliable storage systems having a large capacity. However, in general, in a large-scale storage system using hard-disk drives (in the following, referred to as “HDDs”), the amount of power consumption increases in proportion to the storage capacity. That is to say, the possession of a storage system with a large capacity means an increase in the total cost of ownership including electricity charges. In view of such situations, a technique for reducing the power consumption of HDDs by a cache management algorithm has been proposed (Non-patent document; ZHU, Q., DAVID, F., ZHOU, Y., DEVARAJ, C., AND CAO, P., “Reducing Energy Consumption of Disk Storage Using Power-Aware Cache Management”. InProc. of the 10th Intl. Symp. on High Performance Computer Architecture (HPCA-10) (February 2004)). Also, the problem is not limited to the electricity charges. In general, the floor area for installation increases as the capacity of a storage system increases. This also increases the total cost of ownership.
Incidentally, a flash memory attracts attention as a nonvolatile medium in recent years. A flash memory commonly consumes less than one several-tenth of power when compared with an HDD, and can be read at a high speed. Also, a flash memory is small sized unlike an HDD having a mechanically driven part.
However, a flash memory has a restriction on the number of writing times because of the physical constitution of a cell for holding information. Against such a restriction, the number of writing times of a flash memory has been improved by a technique called wear leveling, in which the number of writing times to each cell is controlled to be averaged by having correspondence between an address to be shown to the upper apparatus and a cell position. In this regard, in the following, an element for holding information is simply called a “flash memory”, and a device including a mechanism for performing the above-described wear leveling, protocol processing for the upper apparatus, etc., is called a “flash memory device”. Although some improvements have been made against the restriction on the number of writing times as a flash memory device by such a technique, there still exists a restriction on the number of writing times of a flash memory device. Also, in addition to that, there is a drawback in that the throughput of a flash memory becomes similar to an HDD when an operation called erase becomes necessary at the time of writing to a flash memory.
As a technique for constituting a storage system using such a flash memory, there is, for example, Japanese Unexamined Patent Application Publication No. 6-324815. In this patent document, a technique in which frequently-accessed parity data is stored in a semiconductor memory such as a flash memory in a RAID configuration, etc., in order to improve performance of a storage system is described. However, means for preventing a restriction on the number of writing times as a storage system has not been disclosed. Also, one RAID group is constructed by mixing an HDD and a flash memory, that is to say, one virtual device is constituted, and thus a virtual device is not controlled in consideration of the characteristics of individual media.